The present invention relates generally to the field of Global Navigation Satellite Systems (GNSS) for navigation, such as the Global Positioning System (GPS) or Galileo.
In a GNSS system, satellites in orbit around the Earth broadcast signals that can be received by a receiver equipped to detect them. The signals are transmitted such that a receiver can determine, under adequate signal conditions, the time of transmission of the signal. By receiving four such signals, the receiver can determine the local time and its own position in a three-dimensional coordinate system.
Such systems often rely on the ability to determine the distance between a satellite and a receiver. By knowing three such distances, an accurate estimate of position can be made by determining the most likely intersection point of three spheres, each centered on a particular satellite and having a radius equal to the measured distance from the satellite to the receiver.
In order to make such a position determination, it is of course necessary for the receiver to have an estimate of each satellite's location. Such location estimates are generated with the help of satellite ephemeris data, which is data that is usable to estimate a satellite's position.
Such satellite ephemeris data is generally stored in the form of parameters which can be used in a known model of the satellite's orbit to generate a position estimate as a function of time. For example, the ephemeris calculations for GPS are described in the document GPS-ICD-200C which is hereby incorporated by reference.
Ephemeris data in the GPS system, for example, are broadcast by each satellite and come in two forms. First, GPS satellites broadcast “almanac data”, which are a form of ephemeris data, although they are not generally referred to as such in the art. Almanac data are intended to be used to determine a rough estimate for a satellite's position for a period of more than a few hours after downloading. This can be useful for the receiver, for example, to determine which satellites are in the receiver's field of view, if the receiver also has a rough estimate of its own position. Almanac data are typically accurate for a few months to within a few kilometers.
GPS satellites also broadcast a more precise set of data referred to as “ephemeris data”. GPS ephemeris data are typically accurate for a few hours to within a few meters. After a few hours, the accuracy of this data degrades rapidly, however.
Both GPS almanac and GPS ephemeris data are broadcast by each satellite as part of a navigation data message that is superimposed upon a CDMA pseudorandom noise code. The ephemeris data for a particular satellite take typically (open sky—no visibility interruption) 30 seconds to download. They cannot be downloaded if the signal is weak (less than −145 dBm). In weak signal conditions, ephemeris data are typically transmitted to the (A)-GPS receiver from an assistance network. Under low signal power conditions, such as those experienced indoors, it is often very difficult to recover the GPS navigation message and thus to recover GPS ephemeris data. This means that GPS receivers which have not been allowed a clear view of the sky for several hours will experience a degradation in their position solution accuracy based on inaccurate estimates of each satellite's current position.
Thus, when a vehicle with a PND (Portable Navigation Device) or GPS receiver mounted on the dashboard or the windshield is parked overnight in an indoor garage, the GPS functions usually cannot deliver a position until quite a long time after the vehicle leaves the garage the next day. The ephemeris data are outdated because of the GPS OFF duration of more than 4 hours (the usual maximum validity period for broadcast ephemerides). Here the term “valid” means producing an error within a preset tolerance. The GPS section takes a long time (up to 10 minutes) in urban canyon environments to recover the broadcast ephemerides from the satellites, due to the low signal level conditions, and the often highly interrupted nature of the received signals. No fix is possible until the broadcast ephemerides are recovered, even if the satellites are tracked.
Thus there is currently a need to extend and improve the accuracy of ephemeris data and the length of time during which ephemeris data can provide an accurate solution.